This invention relates to the reprocessing of irradiated fuel, as well as to a method for dissolving metal oxides in ionic liquids and to novel products or compositions of matter comprising ionic liquids.
By way of example, the irradiated fuel may be that resulting from the use of a fuel assembly in a light water reactor (LWR). Reference will be made below to such fuel but it should be understood that the invention is not restricted to the reprocessing of any particular type of irradiated fuel.
The irradiated fuel from an LWR is located within a Zircaloy cladding which has become oxidised as a result of the irradiation. In the known PUREX process for reprocessing irradiated fuel, the first stage involves the shearing and chopping of the fuel rods so that the irradiated fuel itself can be dissolved in nitric acid.
Molten salts are known for their use as solvents and they have in fact been proposed for use in the reprocessing of irradiated fuels from LWRs. These molten salts are typically mixtures of salts which are liquid only at high temperatures and offer little advantage as solvents over aqueous or organic media.
Recently, a salt, mixtures of salts, or mixtures of components which produce salts, which melt below or just above room temperature have become known. (In the terms of this invention, a salt consists entirely of cationic and anionic species). Such liquids are known as xe2x80x9cionic liquidsxe2x80x9d although this term can be used for salts which melt at relatively high temperatures, including for example temperatures of up to 100xc2x0 C. Common features of ionic liquids include a zero vapour pressure at room temperature, a high solvation capacity and a large liquid range (for instance, of the order of 300xc2x0 C.).
Known ionic liquids include aluminium(III) chloride in combination with an imidazolium halide, a pyridinium halide or a phosphonium halide. Examples include 1-ethyl-3-methylimidazolium chloride, N-butylpyridinium chloride and tetrabutylphosphonium chloride. An example of a known ionic liquid system is a mixture of 1-ethyl-3-methylimidazolium chloride and aluminium(III) chloride.
E. S. Lane, J. Chem. Soc. (1953), 1172-1175 describes the preparation of certain alkylpyridinium nitrate ionic liquids, including sec-butylpyridinium nitrate. No use of the liquids is mentioned but reference is made to the pharmacological activity of decamethylenebis(pyridinium nitrate).
L. Heerman et al., J. Electroanal. Chem., 193,289 (1985) describe the dissolution of UO3 in a system comprising N-butylpyridinium chloride and aluminium(III) chloride.
WO 96/32729 teaches that oxide nuclear fuels may be dissolved in a fused alkali metal carbonate to produce a compound which may be further processed so as to extract uranium therefrom.
WO 95/21871, WO 95/21872 and WO 95/21806 relate to ionic liquids and their use to catalyse hydrocarbon conversion reactions (e.g. polymerisation or oligomerisation of olefins) and alkylation reactions. The ionic liquids are preferably 1-(C1-C4 alkyl)-3-(C6-C30 alkyl) imidazolium chlorides and especially 1-methyl-3-C10 alkyl-imidazolium chloride, or 1-hydrocarbyl pyridinium halides, where the hydrocarbyl group is for example ethyl, butyl or other alkyl.
The present invention provides in a first aspect the use of an ionic liquid containing an oxidant to dissolve a metal, optionally in the form of a compound thereof. The oxidant oxidises the metal to a higher oxidation state, which is normally more soluble in the ionic liquid than is the metal in its original oxidation state. More particularly there is provided a method of dissolving in an ionic liquid a metal in an initial oxidation state below its maximum oxidation state, characterised in that the ionic liquid reacts with the metal and oxidises it to a higher oxidation state.
As used herein, the term xe2x80x9cmetalxe2x80x9d includes not only metallic elements in the (0) oxidation state but also metals in an oxidation state greater than zero, bonded to other elements, for example U(IV) and U(VI). Thus, the metal in its original oxidation state may comprise a metal compound, for example a metal oxide.
The metal preferably comprises uranium (typically as UO2 and/or U3O8) or plutonium (typically as PuO2), or both, and usually fission products. The UO2 or PuO2 is not directly dissolved in the oxidising ionic liquid but, rather, the oxide reacts with the ionic liquid to form an oxidised species which dissolves in the ionic liquid. Such preferred dissolution processes may be used in the reprocessing of an irradiated nuclear fuel. The invention also includes the use of the oxidising ionic liquid to dissolve other metal species, for example a zirconium alloy, which may be in the form of cladding of a nuclear fuel rod.
In another aspect, therefore, the invention provides a method of dissolving a metal in less than its maximum oxidation state in an ionic liquid, wherein one component of the ionic liquid is an oxidant to oxidise the metal to a higher oxidation state. The metal is typically in the form of an oxide thereof.
The invention further provides an ionic liquid comprising an agent to increase the oxidising power of the liquid, for example to make a non-oxidising liquid to be oxidising. Thus, the liquid will contain not just one anion and one cation but in addition another component which enhances the ability of the liquid to react to oxidise a substrate. In preferred embodiments, the liquid contains both the mildly oxidising anion [NO3] and an acid, which may be a Bronsted or Franklin acid such as HNO3, H2SO4 or [NO+], e.g. from [NO][BF4]. The acid makes the liquid more oxidisingly reactive towards various substrates, such as UO2 and PuO2, for example. Thus, the invention includes an ionic liquid capable of oxidising UO2 to convert the uranium to U(VI), especially an ionic liquid which comprises nitrate anions and nitronium cations in concentrations sufficient to enable the liquid to react with UO2 and oxidise the uranium to U(VI), the ionic liquid also containing tetrafluoro-borate(III) and an organic cation. A preferred product is an ionic liquid containing [NO+], typically having been added to the liquid as [NO][BF4].
The products described in the preceding paragraph may often be viewed in a notional sense as comprising an ionic liquid base to which has been added an agent to increase the oxidising reactivity of the liquid. The nature of the notional xe2x80x9cbasexe2x80x9d ionic liquid is not critical to the invention but preferred liquids comprise nitrate anion and an organic cation, especially nitrogen heterocycles containing a quaternary nitrogen, such as pyridinium or substituted imidazolium ions, for example. Exemplary ionic liquids include 1-butylpyridinium nitrate, 1-octylpyridinium nitrate, 1-butyl-3-methylimidazolium nitrate, 1-hexyl-3-methylimidazolium nitrate and 1-octyl-3-methylimidazolium nitrate.
The invention provides in addition novel nitrate-based ionic liquids, including those containing imidazolium and phosphonium cations as well as those containing pyridinium cations other than such cations disclosed by E. S. Lane.
Particularly preferred and novel ionic liquids are 1-butylpyridinium nitrate and 1-octylpyridinium nitrate. These products themselves, free from any oxidation enhancer, forms one aspect of the invention. To avoid ambiguity it should be stated that systematic names are used herein for individual compounds or moieties, i.e. xe2x80x9cbutylxe2x80x9d here refers to the group sometimes called n-butyl (CH3xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94).
The invention also includes ionic liquids which are compound mixtures of ionic liquids, for example ternary liquids, the constituent liquids of which can in combination achieve dissolution (typically by oxidising reaction) of inter alia selected substrates or substrate concentrations which the individual liquids cannot so achieve.
The invention includes further the use in a method for reprocessing an irradiated fuel of an ionic liquid to dissolve the fuel, as well as reprocessing methods which include the step of dissolving the fuel in an ionic liquid.
The present invention, therefore, relates to the use of ionic liquids as solvents, optionally after first serving as a reactive medium.
The Solvent
The solvent comprises an ionic liquid which usually contains an agent or species to make the solvent oxidisingly reactive to inter alia selected substrates, although this agent is not necessarily present in all aspects of the invention (as explained below under the heading xe2x80x9cThe Metalxe2x80x9d). The agent may be an oxidant dissolved in a non-oxidising liquid or an auxiliary agent to increase the oxidising reactivity of another oxidising species. If the solvent contains nitrate ions, the agent increases the oxidising reactivity of the solvent beyond that which would be provided by the nitrate ions themselves; as described above such agents include Bronsted and Franklin acids.
The solvent may in principle comprise any ionic liquid but the liquid normally comprises nitrate anions.
The cation will in practice comprise one or more organic cations, especially nitrogen heterocycles containing quaternary nitrogen and more especially N-substituted pyridinium or N,Nxe2x80x2-disubstituted imidazolium. The substituents are preferably hydrocarbyl and more preferably alkyl, which may be branched, for example. The hydrocarbyl (e.g. alkyl) groups usually contain from 1 to 18 carbon atoms and some usually from 1 to 8 carbon atoms.
The cation may therefore be a disubstituted imidazolium ion where the substituent groups take the form CnH2n+1 for 1xe2x89xa6nxe2x89xa68, and the substituent groups are linear or branched groups. In preferred disubstituted imidazolium ions one substituent has n=1, 2 or 3 (of which methyl is particularly preferred) and the other has n=4, 5, 6, 7 or 8 (of which octyl, hexyl and more particularly C4 especially butyl are preferred). Linear groups are preferred. Alternatively, the cation might be a substituted pyridinium ion, where the substituent group also takes the form CnH2n+1 for 1xe2x89xa6nxe2x89xa68, and the substituent groups are linear or branched groups; suitable substituents include butyl, 2-(2-methyl)propyl, 2-butyl and octyl but straight chain alkyl, especially butyl, is preferred.
Of course, minor quantities of contaminants may be present, e.g. methyl imidazolium in 1-butyl-3-methyl imidazolium.
It will be appreciated from the above that the ionic liquids may be nitrate-based, i.e. have nitrate as anion. Ionic liquids comprising nitrate are new and included in the invention except for certain alkylpyridinium nitrates and polymethylenebis (pyridinium nitrate) compounds taught by Lane. Also novel is the use of nitrate-containing ionic liquids as a reactive medium or solvent. The ionic liquids of the invention comprise nitrate and a cation component which is not exclusively an alkylpyridinium nitrate or a polymethylenebis (pyridinium nitrate). However 1-butylpyridinium nitrate is a particularly preferred ionic liquid which is novel and is also included in the invention. Products comprising the new ionic liquids form an aspect of the invention.
The new nitrate-based ionic liquids may be prepared by mixing aqueous silver(I) nitrate together with an appropriate organic halide. By way of example, one such ionic liquid is prepared by mixing together solutions of aqueous silver(I) nitrate and 1-butyl-3-methylimidazolium chloride (bmim). Silver chloride is precipitated and the liquid 1-butyl-3-methylimidazolium nitrate is formed:
Ag[NO3](aq)+[bmim]Cl(aq)xe2x86x92AgCl(s)+[bmim][NO3](aq)
The product may be purified by filtration and removing excess water from the filtrate.
1-hexyl-3-methylimidazolium nitrate is prepared by a similar method and this material is also a liquid at room temperature.
Alternative cations to pyridinium and imidazolium include quaternary phosphonium cations, e.g. tetra(hydrocarbyl) phosphonium. Suitable hydrocarbyl groups are as described above in relation to pyridinium and imidazolium cations.
The agent to increase the oxidising power of the ionic liquid (when usedxe2x80x94see below under the heading xe2x80x9cThe Metalxe2x80x9d) is typically a Bronsted acid (e.g. HNO3 or H2SO4) or a Franklin acid, for example [NO+], serving in either case to make nitrate more oxidisingly reactive towards substrates such as, for example, UO2 and PuO2. In other words, one class of ionic liquids of the invention contains an oxidant comprising nitrate and a promoter therefor. The oxidant when combined with the ionic liquid may react with the ionic liquid to create a new species which is also an ionic liquid. Thus, [NO][BF4] is believed to react with the nitrate salts of organic cations to form the tetrafluoroborate(III) salt of the cation. An exemplary reaction is:
[Bu-py][NO3]+[NO][BF4]xe2x86x92N2O4+[Bu-py][BF4]
wherein Bu-py is 1-butylpyridinium. [Bu-py][BF4] is novel and included in the invention. The result of the reaction is a ternary ionic liquid. The invention includes the use of other compound ionic liquids.
The reaction of a tetrafluoroborate(III) salt and an ionic liquid results in an anhydrous tetrafluoroborate(III) product. The preparation of tetrafluoroborate(III) salts by such reactions is novel and included in the invention; such a preparative technique finds particular application in the making of organic tetrafluoroborate(III) salts, for example imidazolium, pyridinium and phosphonium salts.
Accordingly, the ionic liquid may comprise organic cations as described above and tetrafluoroborate(III) anions, e.g. be the tetrafluoroborate(III) salt of a quaternary nitrogen-containing heterocycle.
The Metal
The identity of the metal is not critical to the invention. In one aspect the metal prior to dissolution is in a relatively low oxidation state and the ionic liquid composition is oxidising. Preferably, the metal prior to dissolution is in a state less soluble in the ionic liquid than when it is in a higher oxidation state and the ionic liquid composition is oxidising. The metal is typically in the form of an oxide. Preferred metal oxides include a variety of oxides of uranium and plutonium(IV) oxide. Thus, by way of an example only, UO2 may be reacted with an ionic liquid which oxidises the uranium(IV) species to uranium(VI) species, e.g. oxidises uranium dioxide to trans-dioxouranium(VI) in complexed form. Similarly, plutonium(IV), normally as PuO2, may be reacted with an ionic liquid which oxidises the plutonium(IV) to plutonium(VI), e.g. oxidises plutonium dioxide to trans-dioxoplutonium(VI) in complexed form.
In one class of embodiments, the metal oxides comprise plutonium and uranium oxides, primarily in the form of irradiated nuclear fuel, for example an irradiated fuel rod. Nuclear fuel rods consist of fuel pellets contained in cladding and the invention contemplates that the cladding is removed by the oxidising ionic liquid. The cladding is usually a zirconium alloy, for example that sold under the trade mark Zircaloy. In another embodiment of this invention, therefore, the ionic liquid may be used to dissolve an elemental metal (which expression includes alloys), which may be cladding material or may be irradiated metal fuel, e.g. uranium metal which contains fission products and actinides and which started its life either as pure uranium metal or an alloy of uranium and at least one other metal.
In some aspects, the invention relates to ionic liquids which do not contain an acid or other oxidation promoter. Thus, nitrate-based ionic liquids without an additional acid may be used as a reactive medium or a solvent. For example, they may be used as a reactive medium to oxidise substances capable of oxidation by nitrate. A suitable oxide for dissolution in such nitrate-based solvents might include and thorium(IV) oxide.
The Method
The invention is not restricted as to the manner in which the metal is dissolved in the ionic liquid solvent. Normally, the dissolution is performed at an elevated temperature of 50xc2x0 C. or more, e.g. of up to 350xc2x0 C. Most preferably, the elevated temperature is from 50xc2x0 C. to 100xc2x0 C. The metal is normally dissolved with the aid of agitation, typically stirring.
The resultant solution may be further processed, for example for the selective removal of particular species. In particular, uranium and plutonium may be separated from each other by such selective removal techniques; alternatively, a mixed uranium/plutonium oxide may be separated from other components of the solution. Known electrodeposition techniques may be used to extract uranium and/or plutonium species from the solution, whereby fission products remain in solution and the solution may then be subjected to pyrohydrolysis to provide oxides ready for disposal. Other chemical processes may be used to provide a stable waste form for disposal. Alternatively, a method similar to the known PUREX process involving solvent extraction techniques may be used after the initial dissolution of the fuel in an ionic liquid. In this method, the fuel, and preferably the cladding, is dissolved in an ionic liquid and several extraction stages are carried out to remove fission products from the ionic liquid system and to separate the uranium product from the plutonium product ready for their subsequent reuse.
The invention preferably relates to the reprocessing of irradiated nuclear fuel. In one method, a fuel rod is placed in an oxidising ionic liquid and first cladding and then uranium and plutonium are dissolved in the ionic liquid. The uranium and optionally the plutonium are recycled into new fuel rods, e.g. by a method known per se, after being extracted from the ionic liquid. One class of methods includes the step of rupturing the cladding mechanically to expose the fuel pellets to the ionic liquid. In another method, the fuel rod is placed initially in a first ionic liquid for dissolving the cladding and subsequently in a second ionic liquid for dissolving the uranium and plutonium. The uranium and plutonium are normally as oxides thereof.
Those methods of the invention which concern the reprocessing of nuclear fuel may comprise performing one or more steps to process the dissolved fuel to form an intermediate or final nuclear fuel product, e.g. a gel, a powder, a pellet, a fuel rod or a fuel assembly.
The invention may be used in the reprocessing of any irradiated fuel, for example LWR, fast reactor and metal fuels. It may also be used to obtain purified dioxouranium(VI) nitrate from uranium ore or uranium ore concentrate (xe2x80x9cyellow cakexe2x80x9d).